1. Lesson 6.8 Intro to Acids and Bases and Lab: Preparing Solutions
Thurs 4-28 and Mon 5-02
Lesson 6.8 Intro to Acids and Bases and Lab: Preparing Solutions
Mrs. Wilson

2. Objectives
Calculate pH, pOH, [H+] and [OH-] for solutions of strong acids and strong bases.
Prepare solutions, obtain their absorbance values, and use them to determine the molarity of an unknown.
Homework: Lesson 6.9 homework; Lesson 6.9 Daily Quiz next class; Lab due Wednesday 5-11

3. A. The Ion Product Constant, Kw
H2O(l) + H2O(l)  H3O+ (aq) + OH- (aq)
The molarity of the hydronium and hydroxide ions present in a solution always multiply to give you a constant, Kw.
Equation #1:
­Kw = [H3O+][OH- ] = 1.0 x 10-14
Kw is small because only a very small fraction of H2O molecules ever dissociate.
If [H3O+] > [OH-], the solution is acidic.
If [OH-] > [H3O+], the solution is basic.
If they are equal (that is, both are 1.0 x 10-7 M – the solution is neutral.

4. b) [OH-] = 2.7 x 10- 4 b) [H+] = 3.6 x 10-5 Pg. 22
Review:
1. Find [H+] for solutions having the Calculate [OH-] of a solution
following [OH-] value in molarity: when [H+] has the following
values in molarity:
a) [OH-] = 1 x a) [H+] = 1 x 10-3
b) [OH-] = 2.7 x b) [H+] = 3.6 x 10-5
**Then state if each solution is acidic, basic, or neutral.

5. Lesson 6.9 pH and pOH Calculations
B. The pH Scale
pH and pOH scales go by powers of 10
An increase of 1 unit means the concentration of H+ increases tenfold and the concentration of OH- decreases tenfold.
This means something with pH of 3 is 10 times more acidic than something that is pH 4, and something with a pH of 7 is 10 times less basic than something with a pH of 8.

6. B. The pH and pOH Scale pH = -log [H+] and [H+] = 10 – pH
Equations #2, #3 and #4
pH = -log [H+] and [H+] = 10 – pH
pOH = -log [OH-] and [OH-] = 10 – pOH
pH pOH = 14
Note:
H3O+, hydronium ion, can be treated the same as if it were H+.

7. The number of sig figs in the molarity
A Note on Sig Figs
Yes, this is real…
The number of sig figs in the molarity
= The number of decimal places in the pH
Ex. If molarity of [H+] = 1.00 x 10-3 M (3 sig figs)
Then pH = - log (1.00 x 10-3) = 3.000

8. Pg
Calculate the pH and pOH for each of the following solutions, a) – d). All
given values have the unit M.
a) [H+] = 1 x 10-3
c) [OH-] = 2.7 x 10- 4

9. Pg
2. For each of the following pH or pOH values, calculate corresponding [H+] and [OH-]. a) pH = c) pOH = 6.942

10. Pg. 23
Be careful! If you place one drop of hydrochloric acid with a concentration of [H+] = 1.0 x 10-2 M into a full barrel of plain water, what will be the resulting pH?

11. Pg. 23
Always write an ionization equation (for acids) or dissociation equation (for bases) if you are not directly given the hydronium or hydroxide ion concentration. Simple stoichiometry will help you figure out the ion concentration you need. 4. What is the pH of a solution that contains 25 grams of hydrochloric acid (HCl) dissolved in 1.5 liters of water?

12. Pg. 23
#6. What is the molarity of a calcium hydroxide solution that has a pH of 10.07?

13. Preparing Solutions Lab
Prepare solutions of copper (II) sulfate pentahydrate, starting either from solid or by diluting a prepared solution.
Copper (II) sulfate pentahydrate is CuSO4 · 5 H2O.
Use a spectrophotometer to measure each solution’s ability to absorb red light. The absorbance of each solution is directly related to its concentration (molarity).

14. Before You Start the Lab…
How to hold your cuvette:
Hold them by their raised sides ONLY
Wipe the smooth side with a wipe. Your fingerprints will block light absorbance.
Always make sure the cuvette is dry on the outside.
2. Calibrate your colorimeter
Add tap water to a clean cuvette.
Put a lid on the cuvette.
Place the filled cuvette in the sample holder. The arrow marks the point of light so a smooth side should be facing the arrow.
Push the arrow until 635 nm (red light) is lit.
Push the CAL button until it reads 0.

15. THINGS YOU MUST DO
Calibrate your spectrophotometer with plain water in a cuvette.
Make the solution from question #2 in the volumetric flask. Transfer to the appropriately labelled beaker.
Fill a cuvette 2/3 of the way up with the #2 solution; put in spectrophotometer; record absorbance.
Make the solution from question #3 in the volumetric flask. Transfer to the appropriately labelled beaker.
Fill a cuvette 2/3 of the way up with the #3 solution; put in spectrophotometer; record absorbance.
Make the solution from question #4 in the volumetric flask. Transfer to the appropriately labelled beaker.
Fill a cuvette 2/3 of the way up with the #4 solution; put in spectrophotometer; record absorbance.
All waste copper solution goes into the large beaker near the balances.
Get “unknown” concentration solution; put in spectrophotometer; record absorbance. Determine molarity of unknown; show all work.

16. Doing the Lab
Concentration (M)
Absorbance at 635 nm
0.019 M
0.0397
0.038 M
0.0897
0.075 M
0.165
0.150 M
0.344
Your group MUST get questions #1-4 completed on your lab before you start making solutions.
Waste copper (II) sulfate goes into the large waste beaker, not down the sink.
I have the unknowns – let me know when you’re ready for them.